As electronic devices become multi-functional, achieve high speed, and are highly integrated, electro-magnetic interference (EMI) due to electromagnetic radiation emitted from large scale integration (LSI) mounted thereon is problematic. Rigid international standards for EMI exist for, in particular, LSI for vehicle devices and LSI for industrial devices, as well as an LSI mounted on consumer equipment, and LSI that fails to meet the requirements cannot be mounted on vehicle devices or industrial devices.
As one of countermeasures for EMI, a spread spectrum clock generator (SSCG) circuit is known. An EMI peak may be suppressed by modulating an oscillation frequency of a clock signal using the SSCG circuit, compared with a case of fixing the oscillation frequency of the clock signal.
FIG. 20 is a block diagram illustrating a related-art clock signal generating circuit (general SSCG circuit). A clock signal generating circuit 500 illustrated in FIG. 20 is a circuit for receiving a reference clock to output a modulation clock, and includes a reference divider 501 (dividing ratio: 1/R), a programmable divider 502 (dividing ratio: 1/N), a frequency phase comparator 503, a charge pump 504, a filter 505, a V-I converter 506, and a voltage controlled oscillator 507.
However, many circuit blocks are needed when configuring the clock signal generating circuit 500. On this account, it is difficult to implement an SSCG circuit in an LSI that does not allow for an increase in a circuit area.